Please enable Java for an interactive construction (with Cinderella).

This is an example of a false proof. This proof may seem sound when accompanied by an innacurate drawing. But the above picture clearly shows the error of the proof.

Proof: All triangles are isosceles

M is the midpoint of AB. P is the intersection of the perpendicular of AB and the angular bisector of angleBAC. The green lines are the perpediculars of AB and AC.

Given: BM = MC
angleBMP = anglePMC = pi/2
angleBAP = anglePAC

Therefore: triangleANP = triangleAQP

The error in this proof is in the assumption that the bisector and the perpendicular will always meet at a point interior to the triangle. As can be seen here, the point P is outside the triangle, even when the points are moved to make different triangles.

Created with Cinderella

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